1. Field of the Invention
The present invention relates to an apparatus for detecting unwanted particles adhering to surfaces of substrates to be inspected, such as reticles and masks used for printing circuit patterns on semiconductor wafers, for example, in the process of producing LSI (large-scale integrated) product wafers, on which circuit patterns are formed, or substrates for use in liquid crystal displays and more particularly an apparatus and method of specifying a size and location where the particles are adhered to enable a visual inspection.
2. Description of Related Art
An apparatus for detecting particles, in which an inspection stage for placing a substrate to be inspected thereon is adapted to linearly move between a particle-inspecting position and a particle-observing position. Such apparatus have been used with laser beams having an appointed angle of polarization, incident upon a surface of the substrate to be inspected, while being scanned by means of a beam-scanning mirror. Reflected and scattered beams from the surface of the substrate to be inspected are incident upon a detector optical system to detect any existence of particles on the surface of the substrate and to further measure a size of the particles on the basis of the results of detection of the reflected and scattered beams. A particle-observing position where the detected particles can be observed and confirmed by means of a microscope has also been known.
A problem has existed in that some inclination may be produced in a flat surface of the inspection stage to even a slight extent though the tolerance of fabricating accuracy and an assembling accuracy of parts. When the substrate to be inspected is placed on an inspection stage having such an inclination, the substrate to be inspected is also inclined. Generally, a substrate to be inspected, which is an object to be observed, has been considered to be uniformly flat or horizontal and the focus of a microscope has been adjusted at merely one point on the substrate to be inspected, so that the same focus condition must be shifted for other particles because the focus has been adjusted for only one particle position, if the substrate to be inspected is moved when the particles detected by the application of the laser beam are enlargedly observed and confirmed by means of the microscope.
In order to prevent a problem of a focus shift, the focus has been further detected to a high level by an optical method but such an apparatus has been complicated in construction and control.